The best bet would be to edit your /etc/host.allow and /etc/hosts.deny files to control access. A prerequisite is to make sure tcp wrappers in enabled in your /etc/vsftpd.conf file :
# TCP Wrappers
Deny all incoming connections by default in /etc/hosts.deny by adding:
Specify the allowed connection in /etc/hosts.allow by adding:
The above example only allows ftp access from the local network ( 192.168.0.* ) and the IP 188.8.131.52
If there are no /etc/hosts.allow|deny files then you can just create them but make sure that they are rw only by root.